Because of the large temperature difference between the superconductor element and the equipment to be connected to said element, i.e. between ambient temperature and cryogenic temperature which may be about −200° C., it is necessary to interpose a connection structure between the element and the equipment in order to make the temperature transition while minimizing heat losses, and while also complying with electrical constraints due for example to the high voltage of a cable. The structure then includes an electrical bushing constituted mainly by a central conductor surrounded by an insulating sheath for transporting the electricity from the superconductor cable to an outlet connection at ambient temperature. Over some reasonable length, the structure must make the temperature transition while ensuring that losses by thermal conduction are low so as to avoid boiling the cryogenic liquid cooling the cable and/or so as to avoid increasing the costs of cooling the cable.
The solution to the problem consists in providing the connection structure with an adiabatic intermediate enclosure, an airlock or “buffer” enclosure so to speak, placed between the portion at cryogenic temperature and the portion of the connection structure that is at ambient temperature. The electrical bushing passes through the intermediate enclosure. This solution is described, for example, in European patent application EP 1 283 576. The side walls of the intermediate enclosure are constituted by the side walls of a cryostat. The bottom and top walls have fastener flanges through which the electrical bushing passes, the bottom wall being adjacent to the portion at cryogenic temperature and the top wall being adjacent to the portion at ambient temperature. The intermediate enclosure is either evacuated or else filled with a gas. It is therefore essential to ensure that the places where the electrical bushing passes through the bottom and top walls are leaktight, thus leading to manufacturing constraints that are difficult and expensive. For example, even a very small amount of leakage between the portion at cryogenic temperature and the intermediate enclosure (e.g. a leak of about 10−8 millibars per liter second (mbar/L.s)) inevitably leads to a change in the composition of the gas or to degradation in the level of the vacuum in the intermediate enclosure. If the cryogenic fluid is liquid nitrogen, a leak leads to gaseous nitrogen being present in the intermediate enclosure, thus leading firstly to an additional consumption of liquid nitrogen, and secondly to a reduction in the thermal insulation of the intermediate enclosure. The excess pressure in the intermediate enclosure that results from such a leak is incapable of being controlled by means of safety valves since opening a valve would destroy the thermal insulating medium (vacuum or gas). In addition, it is not easy to perform on-site maintenance of such a connection structure away from the workshop. For example, reestablishing a vacuum in the intermediate enclosure or refilling it with gas on site requires special equipment and specially-trained staff.